The present invention relates generally to sailing craft.
Conventional sailing boats use movable mass or buoyancy to balance the capsizing moment caused by the sail force. This mass may be part of the keel, the actual crew on the windward side of the boat, water pumped between tanks, or many other methods which move the centre of mass to the windward side of the boat. Buoyancy moves as the craft tilts and more water is displaced on the leeward side of the hull, or the leeward hull in the case of a multi-hulled craft. Under steady state conditions the three moments must be balanced.
Absolute stability may only be achieved by positioning the sailing craft centre of mass below its centre of buoyancy. This carries a huge weight and wetted area penalty, which makes such craft slow. High speed sailing craft must rely on wide or multi-hull designs and/or movable ballast, usually crew, to achieve stability.
Hydrofoil craft use underwater foils to balance the capsizing moment without moving masses but they do produce the same moment, putting similar stresses on the structure. The total foil force must always be greater than the total craft weight, inducing more parasitic drag in the foils than if the foils were supporting the craft weight alone.
Sailing craft are known which have tilted sail-sails, with the intention of reducing water drag by supporting at least part of the craft on air. The main problem encountered by the most successful of such craft has been instability due to rapid changes in the moments at play. This is also a significant problem with most conventional high speed sailing craft.
All prior art sailing craft achieve equilibrium by balancing large and often rapidly changing moments. High speed sailing craft, as discussed above, cannot be inherently stable and so must sail at the limit of their ability to balance these moments. The fastest boat in a race is therefore usually the one closest to capsizing.
It is an object of the present invention to provide an improved sailing craft which alleviates one or more of the aforementioned problems.
According to the present invention there is provided a sailing craft including a hull assembly, a keel operatively connected to said hull assembly, a turret assembly operatively connected to said hull assembly for at least partial rotation relative thereto about a rotation axis, the turret assembly when in use being adapted to carry the craft""s crew, a mast operatively connected to and projecting from the turret assembly, a sail assembly operatively connected to the mast in spaced relation from the turret assembly, said sail assembly including a sail member which is movable relative to the mast and which is adapted to catch the wind so as to provide a force for propelling the craft, the sail assembly further including wind responsive means such as for example a vane operable to position the sail member with respect to the wind direction.
Preferably, the arrangement is such that when the craft is in a normal sailing mode the major forces acting on the craft are substantially directed through the region of a single point. Preferably, the sail assembly, mast, turret and crew have a centre of mass which is at or in the vicinity of that region of the single point. The position of the crew on the turret assembly can be changed so that the position of the centre of mass can be changed.
In one preferred embodiment, the hull assembly has a horizontal plane which is generally parallel to the water upon which it floats when in the normal sailing mode, the rotation axis of the turret assembly being generally vertical to the horizontal plane. Preferably, the centre of mass is in the region of the rotation axis of the turret assembly. Preferably, the hull assembly has a centre of buoyancy and the keel is operatively connected to the hull assembly in the region of the centre of buoyancy.
Preferably, the mast has a longitudinal axis which is from 15xc2x0 to 75xc2x0 from the horizontal plane of the hull assembly. Preferably, when in the normal sailing mode the longitudinal axis of the mast passes through the region of the centre of mass.
Preferably, the sail member is operatively connected to the mast for movement relative thereto about 3 axes rotation. Preferably, the sail member is adapted to pitch, roll and yaw with respect to the mast. The sailing craft may further include a rudder for steering the craft.
Preferably, the sail member includes a generally aerofoil shaped body. In one preferred form, the sail includes a frame member with the aerofoil shaped body attached thereto and the wind responsive means being operatively connected thereto. Preferably, the sail includes regulating means such as an elevator which is adapted to change the angle of attack of the aerofoil shaped body.
In one preferred embodiment, the turret assembly includes a main body having opposed end portions, the axis of rotation being disposed between the end portions, the mast being operatively connected at one end portion and the crew support section being disposed towards the other end portion with the axis of rotation being between the mast and the crew support section. The turret assembly may be temporarily prevented from rotation if desired so that the craft is substantially steered by the wind.
According to a preferred form the sailing craft alleviates high speed control problems by directing substantially all major forces acting on the craft through one point or region when in the normal sailing mode. This effectively eliminates the moments which change too rapidly for the helmsman to control when the wind shifts in speed and direction. Wind speed changes on the sailing craft produce only acceleration or deceleration in the intended direction of travel, with no significant tendency to capsize, change course or pitch forward.
A primary benefit of the sailing craft of the invention in its preferred form is that water drag may be reduced by supporting some of the craft weight in a controllable way. The propulsive force available is limited only by sail force and the mass of the boat. At that limit the craft is completely or substantially clear of the water except for the keel and, optionally, the rudder. The resultant low drag allows very high speed.
The fastest way of sailing the sailing craft is to have only its keel in the water. Excessive sail member force will not tend to capsize the craft, but will lift it until the wetted keel area is insufficient to generate enough lift to balance the sail member force. If this happens gradually, the keel efficiency will drop, the boat will lose speed, possibly slip sideways in the downwind direction, and drop lower in the water due to lower apparent wind speed and a consequent reduction of the sail force; that is, it recovers from crew error without a significant penalty like a capsize. If the sail member force rises rapidly the craft becomes completely airborne, it will accelerate sideways (downwind) without the side force of the keel to balance the sail member force side component, lose airspeed as it gets carried with the wind, the severity of the landing depends on the height of the jump, but the skill of the crew, and the design of the craft would make such a manoeuvre possible without damage.
Two automatic operation modes are possiblexe2x80x94steering or sail setting. Firstly, with a conventional rudder turning the hull assembly to the desired course and allowing the sail member to automatically drive the turret assembly to the optimum angle. Secondly, as the turret assembly maintains an almost constant angle to the wind when the sail member is loaded, and resists rotation from its optimum position, the hull assembly may be turned with respect to the turret assembly without using a rudder. The turret assembly pivot may also be temporarily locked to make the craft automatically keep a substantially constant bearing with respect to the apparent wind direction. As the craft changes speed however, it will change bearing with respect to the true wind direction.
The mast of a conventional craft is subject to high and variable loads. The mast and associated structure of the proposed craft is subject to a bending moment due to the weight of the sail member at rest, but this moment remains substantially constant under all sailing conditions provided the mast is substantially in line with the sail member force. The sail member force imposes only a relatively small and substantially tensile stress to the mast. The structure may therefore be made lighter and more flexible than in prior art craft.